1. Technical Field
The present invention relates to a copolymer including a monomer unit derived from an aromatic vinyl compound and a monomer unit derived from a compound containing conjugated diene, a rubber composition including the copolymer, and a pneumatic tire including the rubber composition.
2. Related Art
Conventionally, for example, it is required in a rubber composition used in a tire to balance grip performance on a wet road surface (wet grip performance) and rolling resistance performance contributing to low fuel consumption in high dimension. In general, regarding wet grip performance, tan δ at 0° C. is used as an index, and the performance is excellent as the index is large. Regarding rolling resistance performance, tan δ at 60° C. is used as an index, and the performance is excellent as the index is small. Wet grip performance and rolling resistance performance are incompatible characteristics, and it is not easy to simultaneously improve those characteristics.
JP-A-H09-328577 proposes to add a copolymer resin (petroleum resin) of a C5 fraction by pyrolysis of naphtha and styrene or vinyl toluene to a diene rubber in order to improve wet grip performance without deteriorating rolling resistance performance. However, when a petroleum resin has been added, modulus of elasticity of a rubber composition increases at low temperature, and grip performance (low temperature characteristics) is deteriorated.
JP-A-S61-203145 discloses to add a low molecular weight styrene-butadiene liquid polymer together with a diene rubber in order to improve road surface grip power in a racing tire. However, when the liquid polymer has been added, the decrease of hardness of a rubber composition obtained is large, and the decrease of driving stability when the rubber composition is used in a tire is large.
JP-A-2005-154696 discloses to concurrently use a petroleum resin and a styrene-butadiene liquid polymer, but it is not easy to suppress the deterioration of the above-described low temperature characteristics and driving stability.
JP-A-2014-088544 discloses to use a branched conjugated diene copolymer obtained by incorporating a diene component such as myrcene in a styrene-butadiene rubber as a rubber component of a rubber composition for a tire in order to achieve both wet grip performance and rolling resistance performance. However, this patent document is to incorporate a branched conjugated diene compound such as myrcene as a monomer unit for the purpose of improving characteristics of a styrene-butadiene rubber having high styrene content. The copolymer is used as a rubber component, and therefore has high molecular weight, and it is not easy to suppress the deterioration of low temperature characteristics.